A battery stores electrochemical energy by separating an ion source and an ion sink at differing ion electrochemical potentials. A difference in electrochemical potential produces a voltage difference between the positive and negative electrodes; this voltage difference will produce an electric current if the electrodes are connected by a conductive element. In a battery, the negative electrode and positive electrode are connected by two conductive elements in parallel. Generally, the external element conducts electrons, and the internal element (electrolyte) conducts ions. Because a charge imbalance cannot be sustained between the negative electrode and positive electrode, these two flow streams supply ions and electrons at the same rate. In operation, the electronic current can be used to drive an external device. A rechargeable battery can be recharged by application of an opposing voltage difference that drives electronic current and ionic current in an opposite direction as that of a discharging battery in service. Thus, the active materials of rechargeable batteries need to be able to accept and provide ions.
Redox flow batteries are energy storage devices in which the positive and negative electrode active materials are soluble metal ions in liquid solution that are oxidized or reduced during the operation of the cell. A redox flow battery typically has tanks for separately storing the positive and negative electrode active materials, along with a current-extracting stack that comprises at least positive and negative electrode compartments separated by an ionically conductive membrane, and positive and negative current collectors that facilitate the transfer of electrons to the external circuit but do not participate in the redox reaction (i.e., the current collector materials themselves do not undergo Faradaic activity). In a redox flow battery, the total stored energy may be increased by simply increasing the size of the tanks holding the electrode active materials without increasing the amount of other components such as the separator, current collectors, and the like. However, while conventional redox flow batteries possess many attractive features, such as scalability and the ability to decouple stored energy (tanks) from power (stack), they typically have relatively low energy densities. Thus, there is a need for improved, high-energy-density energy storage devices.